Thanks for mentioning Beancount; I didn't know about it before. I'm American but have a job in a different country, so I deal with two currencies routinely, and haven't found a good way of handling multiple currencies in Gnucash. So my wife and I have been keeping our records in text files. I'll look into what it would take to switch over to Beancount; I bet I could write a conversion script (or get an LLM to write most of it for me, even) that would do 95% of the work since we've been pretty consistent in our format, and throw up warning messages about any entries that it couldn't parse (we haven't necessarily been 100% consistent).
I just might join you in Beancount + Fava land.